csci 4550/8556 computer networks comer, chapter 5: local asynchronous communication (rs-232)

CSCI 4550/8556Computer Networks

Comer, Chapter 5:Local Asynchronous Communication (RS-232)

Bit-wise data transmission

Data transmission requires: Encoding bits as energy Transmitting energy through medium Decoding energy back into bits

Energy can be electric current, radio, infrared, light Transmitter and receiver must agree on encoding scheme and transmission timing

Asynchronous communication

One definition of asynchronous: transmitter and receiver do not explicitly coordinate each data transmission

Transmitter can wait arbitrarily long between transmissions Used, for example, when transmitter such as a keyboard may not always have data ready to send

Asynchronous may also mean no explicit information about where data bits begin and end

Using electric current to send bits Simple idea - use varying voltages to represent 1s and 0s One common encoding uses a negative voltage for a “1 bit” and a positive voltage for a “0 bit” The following figure illustrates one possible transmission of the data “100101”; observe that the first bit appears rightmost in the figure (it was transmitted first).

Sending bits - example

1 0 1

1 0 1 1

1 0 1 1 0

1 0 1 1 0 1

Transmission timing The encoding scheme leaves us with several questions:

How long will the voltage last for each bit? How soon will next bit start? How will the transmitter and receiver agree on timing?

Standards specify operation of communication systems.This allows devices from different vendors that adhere to the standard to interoperate.Some standards organizations:

• International Telecommunications Union (ITU) • Electronic Industries Association (EIA) • Institute for Electrical and Electronics Engineers (IEEE)• National Institute for Standards and Technology (NIST)

RS-232

RS-232 is a standard for the transfer of characters across copper wire It was produced by EIA.The full name for the standard is RS-232-C.RS-232 defines serial, asynchronous communication:

Serial - bits are encoded and transmitted one at a time (as opposed to parallel transmission) Asynchronous - characters can be sent at any time and bits are not individually synchronized

Details of RS-232

Components of the standard: Connection must be less than 50 feet Data represented by voltages between +15v and -15v 25-pin connector, with specific signals such as data, ground and control assigned to designated pins Specifies transmission of characters between, e.g., a terminal and a modem

Transmitter never leaves wire at 0v; when idle, transmitter puts negative voltage (a 1) on the wire

RS-232 wiring and connectors

Identifying asynchronous characters

Transmitter indicates start of next character by transmitting a zero

Receiver can detect transition as start of character Extra zero called the start bit

Transmitter must leave wire idle so receiver can detect transition marking beginning of next character

Transmitter sends a one after each character Extra one call the stop bit

Identifying asynchronous characters

RS-232 terminology: MARK is a negative voltage (== 1) SPACE is a positive voltage (== 0)

Thus, character represented by 7 data bits requires transmission of 9 bits across the wire :

Timing

Transmitter and receiver must agree on timing of each bit Agreement accomplished by choosing transmission rate

Measured in bits per second Detection of start bit indicates to receiver when subsequent bits will arrive

Hardware can usually be configured to select matching bit rates

Switch settings Software Autodetection

Measures of transmission rates

Baud rate measures number of signal changes per second Bits per second measures number of bits transmitted per second In RS-232, each signal change represents one bit, so baud rate and bits per second are equal If each signal change represents more than one bit, bits per second may be greater than baud rate

Framing

Start and stop bits represent framing of each character If transmitter and receiver are using different speeds, stop bit will not be received at the expected time Problem is called a framing error RS-232 devices may send an intentional framing error called a BREAK

Full-duplex communication

Two endpoints may send data simultaneously - full-duplex communication Requires an electrical path in each direction

RS-232 connection standards

RS-232 specifies use of 25 pin connector (DB-25) Pins are assigned for use as data, ground and control:

Pin 2 - Receive (RxD) Pin 3 - Transmit (TxD) Pin 4 - Ready to send (RTS) Pin 5 - Clear to send (CTS) Pin 7 - Ground

2-3 swap Cable must cross-over wires to connect pins 2 and 3 on receiver and transmitter

To avoid 2-3 swap, RS-232 specifies that modems transmit on pin 2 and receive on pin 3, while computers transmit on pin 3 and receive on pin 2 However, RS-232 cables between two computers must have 2-3 swap

RS-232 cable breakout-box

May need to test RS-232 connections Breakout-box gives access to signals

Limitations of real hardware Effects of wire mean waveforms look like that shown to the right: Longer wire, external interference may make signal look even worse RS-232 standard specifies how precise a waveform the transmitter must generate, and how tolerant the receiver must be of imprecise waveform

Hardware bandwidth

Limitations on time to change voltages imposes upper limit on number of changes per second Theoretical upper limit is called the bandwidth Measured in cycles per second or Hertz

Bandwidth and data transmission Nyquist sampling theorem expresses relationship between bandwidth and maximum data transmission speed For RS-232, using two voltages, maximum speed over medium with bandwidth B is 2 B In general, for system using K different states, maximum is 2 B log2 K

In practice, noise limits maximum data transmission rate to less than maximum allowed by Nyquist sampling theorem.

Shannon’s Theorem

Claude Shannon’s theorem states thatC = B log 2 (1 + S/N)

where C is the effective bandwidth limit (bits/sec), B is the hardware bandwidth, S is the average signal power, and N is the average noise power.Usually, the signal to noise ratio is expressed in decibels (dB), computed as 10 log10 (S/N).

ExampleThe telephone system is usually characterized as having a signal to noise ratio of 30dB, and a bandwidth of 3000 HZ (3kHZ).Since 30 dB is a S/N ratio of 1000, the effective bandwidth in the presence of noise is computed as

3000 log 2 (1 + 1000).

Since the log 2 (1024) = 10, this expression is approximately 30,000 bits per second.This rate can be exceeded only by increasing the signal to noise ratio, or by encoding the data so it requires fewer data bits (which is the technique used by modern modems).

Summary

Asynchronous communication - data can start at any time; individual bits not delineated RS-232 - EIA standard for asynchronous character transmission Characters per second and baud rate Bandwidth limits maximum data transmission rate